Exterior doors sometimes stick and it is always inconvenient. The door furniture might be to blame. Alternatively, it could be the British weather. Either way, you can often free the door yourself with a few simple tools. Failing that, you will have to seek professional assistance.
Unlock the door. Push hard against it if you are standing outside the house. Pull hard if you are standing inside the house. Doors often expand slightly as weather conditions change. This means they sometimes stick in the frame. If this is the reason for the door sticking, a little force should solve the problem.
Slip a credit card, or similar item, between the latch bolt and the strike plate, if you can see the latch bolt has jammed. This is known in the trade as "loiding" the lock, according to woodworker Chip Rosenblum. If you have a mortice lock, sometimes, the retractable latch bolt mechanism breaks, leaving the bolt jammed in the strike plate or keep box. This will cause the door to stick. Wriggle the card into place so it pushes the jammed latch bolt back into the lock body. The stuck door will then open.
Inspect the hinges if you do not have a mortice lock or you are sure it is not jammed. If the hinges have sagged, it could cause the door to stick. Follow the advice of carpenter Peter Brett and knock out the hinge pins. Insert the tip of an old screwdriver with a thin blade against the bottom of one of the pins. Tap the screwdriver handle with a hammer and the pin will start to rise up from the hinge. Proceed in like manner for all the hinge pins. The hinges will now be in two parts and you will be able to open the door. You can only do this from the inside so if you are outside your house, it is time to call a locksmith.